Evohome with basic oil boiler

[antonpiatek]

I'm thinking about an evohome setup, however I want to check if I need to change my plubming or electrics first.

My current heating setup has a dual channel timer. One channel fires the oil boiler which is a gravity fed vented cylinder. The second channel triggers the CH pump. No valves are in the system, so to heat the house the timer has to have both the HW and CH come on at the same time.

If I get two evohome remote relays (probably one as part of the hot water kit), can the evohome controller be told to fire the HW boiler whenever the heating is on? Or do I need to make changes to my home heating install in order to have the CH remote relay close both circuits in hardware?

[Little Tinker]

I'm thinking about an evohome setup, however I want to check if I need to change my plubming or electrics first.

My current heating setup has a dual channel timer. One channel fires the oil boiler which is a gravity fed vented cylinder. The second channel triggers the CH pump. No valves are in the system, so to heat the house the timer has to have both the HW and CH come on at the same time.

If I get two evohome remote relays (probably one as part of the hot water kit), can the evohome controller be told to fire the HW boiler whenever the heating is on? Or do I need to make changes to my home heating install in order to have the CH remote relay close both circuits in hardware?

I think that if you're going to the trouble and expense of having EvoHome installed then it would make sense to update the system to at least Honeywell C-Plan (temperature controller gravity HW, pumped CH), and that would happen to give you the ability for a CH relay to fire the pump and boiler (while one on HW will still fire just boiler) while only needing the vale to be inserted in a suitable length of pipework.

It's often not too hard to make HW pumped as well by improve HW cylinder reheat times by switching to S-Plan.

You can see water and electrical schematics here: http://wiki.diyfaq.org.uk/index.php?...ols_and_Zoning.

[antonpiatek]

I take your point, but it would not change the evohome install, and I can do that myself without getting a plumber in. Refitting the system to add a valve is probably something I can't do myself (I'm not even sure I can understand all the pipes currently running around the place as it is, though I honestly haven't spent time looking into it).
The boiler already has a thermostat for the HW cylinder, so other than not having to heat the cylinder to heat the house, I don't see much gain. Both seem to heat up pretty quickly as it is.

[DBMandrake]

I'm thinking about an evohome setup, however I want to check if I need to change my plubming or electrics first.

My current heating setup has a dual channel timer. One channel fires the oil boiler which is a gravity fed vented cylinder. The second channel triggers the CH pump. No valves are in the system, so to heat the house the timer has to have both the HW and CH come on at the same time.

If I get two evohome remote relays (probably one as part of the hot water kit), can the evohome controller be told to fire the HW boiler whenever the heating is on? Or do I need to make changes to my home heating install in order to have the CH remote relay close both circuits in hardware?

What you ask can't be easily done with your current hardware configuration, at least not in a way that isn't kludgy...

Hot water control on the Evohome requires either two or three BDR91's - two gives you a hot water demand relay and a heating demand relay - normally these would be connected to hot water and heating zone valves which would then fire the boiler for you if either had demand.

If you add a 3rd BDR91 you can also have a boiler relay, which can fire the boiler directly rather than relying on switches in the zones valves to do this, and this boiler relay will come on for both hot water and heating demand.

So you could connect the boiler control relay to fire the boiler and the heating relay that normally controls a zone valve to the pump.

Then either heating or hot water demand would fire the boiler but only heating demand would run the pump.

However there are several pitfalls with such an approach:

1) You'd need 3x BDR91's even though the hot water one doesn't have anything to do, because you can have a hot water configuration with boiler relay and hot water relay (no heating zone valve relay) but you can't have a hot water configuration with a boiler relay and a heating relay but no hot water relay - it won't let you set this up. So you'd have to wire up a third "dummy" BDR91 to power and bind it as a hot water relay even though it won't be controlling anything, which is a bit of a waste of money and space.

2) If you use hot water control on the Evohome you must use the supplied temperature sensor - either strap on or insertion - you can't just use the hot water schedule as an ON/OFF schedule controlling a relay and rely on something else to control the temperature - Evohome wants to measure and control the hot water temperature for you. And yet when you do use the temperature sensor it won't be able to stop the hot water from getting too hot if there is also a heating demand - it will gradually climb up to near your flow temperature, but no doubt you have this problem already on an uncontrolled gravity circulation system.

You could convert to a C plan system by fitting a zone valve to the return leg of the indirect loop as Little Tinker says, then connect the previously "dummy" hot water relay to control the zone valve - now when the hot water temperature is satisfied evohome will be able to close this valve (interrupting the gravity circulation) and prevent the hot water getting any hotter even if heating is still in use. This is the simplest way to get a system that "sort of" works properly.

I don't really recommend doing a C Plan conversion and then installing Evohome though. I came very close to doing a C Plan conversion myself - although gas, my original system was very similar to yours - timer for the boiler, wall stat for the pump, gravity circulation hot water, and I'm glad I didn't.

The problem is that gravity circulation is just far too slow and inefficient - it used to take well over an hour for the hot water to heat up on my system using gravity circulation, and sometimes the circulation would not "kick off", so sometimes I would have no hot water. (This can happen if there is too much hydraulic resistance in the indirect loop, like sharp bends or constricting valves) Also when the pump is running for central heating this tends to disrupt gravity circulation.

Yet another problem with this kind of system is that it's not good for the boiler and pump to repeatedly cycle on and off every 10 minutes under TPI control with no pump overrun - this causes localised boiling in the heat exchanger (especially if its old and limescale encrusted) every time the pump stops and the temperature is over about 60 which will cause noises like gurgling, whooshing, crackling in the pipes and radiators after a while, and can easily lead to full blown vapour lock which is very harmful for the pump (it causes the bearings to run dry) and can cause the heating to stop working for several minutes until the vapour lock clears itself.

If the boiler is going to cycle on and off under TPI control there really needs to be at least a few minutes of pump overrun to avoid these problems, and once you add pump overrun it kills the effectiveness of the gravity hot water system because the pump disrupts the convection flow to the cylinder even if it is coming off separate taps on the boiler heat exchanger - a problem I had with mine once I added pump overrun.

In short, this is a fairly compromised arrangement that I wouldn't recommend.

I ended up doing a full conversion from gravity hot water to S-Plan, but I did all the plumbing, fitting of zone valves and wiring myself, and unless you're confident in this sort of work I wouldn't recommend attempting it yourself. It took me about a day and a half of hard work all up (including flushing and cleaning the system) and I'm fairly comfortable with this kind of work and did a lot of research and planning beforehand.

I'm glad I did a full S-Plan conversion though - my hot water can heat up from stone cold to fully hot in less than 25 minutes now, reliably, and tops up the temperature after a few minutes use in as little as 6 minutes compared to 30-40 minutes before, and this is with the original old fashioned cylinder not a fancy modern fast heat cylinder. It shuts off at the correct temperature so I have no problems with overshoot now, and it works seamlessly with the heating so I don't have to worry about the relative scheduling of hot water and heating - I can schedule them completely independently of each other.

My opinion is that it wouldn't be worth trying to integrate Evohome with your existing system configuration, but that you could gain a lot by getting it converted to S-Plan (including fitting an automatic bypass valve which is necessary with S-Plan, and a pump overrun timer) which could be controlled with a standard 2 channel timer such as the one you already have to begin with, and then you could add Evohome with the hot water control kit at a later time to replace the two channel timer and wall stat.

[Little Tinker]

I take your point, but it would not change the evohome install, and I can do that myself without getting a plumber in. Refitting the system to add a valve is probably something I can't do myself (I'm not even sure I can understand all the pipes currently running around the place as it is, though I honestly haven't spent time looking into it).
The boiler already has a thermostat for the HW cylinder, so other than not having to heat the cylinder to heat the house, I don't see much gain. Both seem to heat up pretty quickly as it is.

If you have a cylinder stat it is presumably wired up something like this (from DIYnot):



Given the constraints you outline and the outcome that even with EvoHome cylinder stat and relay you wouldn't actually be controlling HW temperature it seems best to not even try .

So your best (but poor ) way forwards may be to do what I've done, which is adding EvoHome to only the CH side. My HW is still on the old programmer and hardwired cylinder stat loop, with a BDR91 controlling the CH side. I was able to add mine as a boiler controller, maybe because I have HR92 rad valves installed. Without those I think you might add the BDR91 as a zone controller. Either way you should get the right outcome I think by adjusting the "Heating Live" and "Room Stat" parts of that diagram with the following wiring in the BDR91:

1. Mains supply to L and N
2. Link L to point A on relay side
3. Link point B on relay to pump and NC side of cylinder stat

Then when:

1. EvoHome doesn't call for CH the old wiring can still fire boiler for HW.
2. EvoHome does call for CH pump and boiler will fire, but heat will leak to HW as well.